1. Field of the Invention
This invention is generally related to a rotary apparatus having the capability of stretching the longitudinal (travel) dimension of a moving web. More particularly, the invention relates to an apparatus which provides additional engagement of a web whose edges have been initially engaged and partially stretched longitudinally, thereby minimizing scalloping during continued longitudinal web stretching.
2. Prior Art
In order to impart improved physical strength to crystallizable polymeric films, such material may be oriented (stretched) by a preselected amount, in the longitudinal or both the transverse and longitudinal dimensions. With respect to orientation in the longitudinal direction, apparatus of the drafter type are commonly employed. Drafters are typically comprised of a series of feed and take-up rollers whereby successive take-up rollers operate at a higher surface velocity than their associated feed rollers. As a result, webs being transported through are elongated in their direction of travel.
A web being elongated exhibits a tendency to compensate for the increasing shrinkage associated therewith a generally undesirable phenomenon known as "necking". In drafter apparatus, necking may be reduced by engaging the lateral web edges with a series of clamps, typically disposed in an endless arrangement for successive web edge engagement immediately after the moving web leaves a feed roller zone. To permit web elongation, the clamps are free-riding in, for example, longitudinal tracks. As the moving web elongates between the feed and take-up rollers, engaged clamps accelerate freely creating progressively increased spacing between adjacent clamps. Consequently, intermediate edges of the web between adjacent clamps begin to exhibit a secondary necking tendency known as "scalloping". Later trimming to remove the defect is wasteful. To overcome this problem, it is necessary in drafters to disengage the clamps from the lateral edges of the web and introduce the web into a duplicate drafter, having more closely longitudinally-spaced clamps, for continued elongation.
Clamps that are adapted to engage intermediate web edges tending to scallop, as above described, are not practical in conventional drafting. Precisely when and where in the course of web travel adjacent engaged clamps begin to draw apart during web stretching is dependent on a number of factors, such as web temperature and speed difference between feed and take-up rollers. For example, at higher web temperatures, web stretching and drawing apart of the clamps may be more pronounced at an early point between feed and take-up, whereas at lower web temperatures and low speed differences, web stretching and drawing apart of the clamps may take place at another unpreditable and later point. As can be appreciated, the synchronization of additional clamps into a drafter is particularly difficult and the apparatus is subject to breakdown when processing conditions are altered slightly.
In this context, a definite need has arisen for the introduction within one apparatus of additional clamps to secure intermediate web edges of a partially stretched web.
In contrast to drafter longitudinal stretching wherein clamps are freely accelerating in response to and under control of the stretching web, longitudinal stretching in other prior art apparatus is governed by mechanically controlled clamp acceleration. For example, in U.S. Pat. Nos. 2,571,355 and 3,833,973, rotary devices, comprising rotatable members, are described wherein the longitudinal spacing of adjacent angularly spaced clamps carried by the members is fixed by their position within associated radial slots in the members. During rotation of the members, the clamps in each member are cammed or urged radially outward, by eccentric camming surfaces, to positions of greater longitudinal spacing in a controlled and predictable manner. However, the clamps described in such prior art are suitable only for engaging a web edge which is approaching these clamps in a radially inward direction from the periphery of the circular members, typically occurring, for example, when a web first enters the apparatus. In particular, the clamps in these patents are shown as having fixed platens (base plates) which are closer than the approaching web to the axes of rotation of their associated rotatable members. After web contact with the platen of a given clamp, an outward pivotable jaw closes in a radially inward direction toward the plate to engage the web edge disposed therebetween. Thereafter web elongation occurs, during rotation of the members by the clamp camming mechanism, as described. Neither of these patents, however, discloses or suggests how to provide additional engagement of a partially stretched web along its intermediate edges extending between adjacent clamps and wherein the web is moving radially outward, for minimizing scalloping during continued stretching.
According, in dealing with the problem of scalloping along intermediate web edge spans appearing between accelerating adjacent clamps during longitudinal stretching of a web, prior art drafting operations disadvantageously require duplicate apparatus in series to provide engagement of the web at closer web edge intervals. Furthermore, prior art rotary-acting apparatus fail to disclose or suggest the need for additional engagement of a partially stretched web along its intermediate edges.